In known control devices means sensitive to the operating temperature of the motor act upon a valve which, when the motor temperature exceeds a certain level, uncovers an opening through which outside atmospheric air is supplied to the motor-fan unit. Thus, air is conveyed directly to the fan without first passing through the dust container. In this construction and arrangement, the motor is cooled to a normal operating temperature, whereby the risk of overloading and overheating of the motor is eliminated. However, this device is relatively complex and expensive to manufacture. Furthermore, there is a sensing means arranged in the circuit of the motor for turning the motor off when it is operating at too high a temperature. Due to present safety regulations, this solution is also expensive.